Up to now, induction motors have been mainly used as a drive source of mechanical devices, but at the present time, synchronous electric motors using permanent magnets are being adopted from the viewpoint of energy saving and high efficiency. In the case of the synchronous electric motors, an electric motor that does not have a magnetic pole position sensor has advantages that there is no risk of failure of a magnetic pole position sensor and the cost can be reduced.
On the other hand, in the case of a synchronous electric motor (hereinafter abbreviated as an electric motor) without any magnetic pole position sensor, a phenomenon called a synchronization loss phenomenon in which a rotational speed recognized by an inverter which controls the electric motor does not match a rotational speed of an actual electric motor shaft occurs, resulting in a possibility that the electric motor shaft does not rotate and the electric motor does not work.
For example, according to Japanese Unexamined Patent Application Publication No. 2012-60781 (PTL 1), abnormality of a rotation state of the electric motor can be detected by estimation calculation of an axis error of the electric motor.